The exemplary embodiments disclosed herein relate to the use of color patch codes in a printed image for conveying information about the content of the image. More particularly, they relate to patch codes for color calibration job identification encoding, although it is to be appreciated that the exemplary embodiments have other applications.
By way of background, there are two phases involved in the color correction of a color printer: calibration and characterization. Calibration involves maintaining the printer at a defined state, while characterization involves determination of the multidimensional transformation or profile that relates device color signals (e.g., cyan, magenta, yellow, and black) to spectrophotometric or colorimetric signals (e.g., CIELAB color scale). Characterization is performed relatively infrequently, sometimes only once at the factory that produces the printer. Calibration, on the other hand, needs to be performed often to compensate for printer drift and bring the device back to its nominal state. While the term calibration will be used throughout, the concepts also apply equally to the characterization process.
The calibration process involves sending an image with pre-specified device signals (i.e., a target) to the printer, and making spectrophotometric measurements of the print with the use of a spectrophotometric scanner. The device and spectrophotometric signals together are used to build or update the calibration tables. In a production environment, many printers, perhaps thirty to forty, might be going through a calibration process at the same time. In a typical environment, operators must manually keep track of each printed page, and there can be many printed target types printed for each printer. The operator must then feed each page to a spectrophotometric scanner and tabulate results of scanning each target type. The results of each scan must be manually associated to the corresponding printed target, and to the correct printer. Considering the quantity of pages printed and the amount of work necessary to manually track each printed page, there is considerable possibility for error. Pages can be accidentally misordered, and scanning results can accidentally be associated with an incorrect printed target or printer. This can result in highly inaccurate calibrations, and calls upon the difficult task of diagnosing the errors.
When more than one page is used in color calibration, such as when calibrating multiple printers, or using multiple sheets per printer, there is the opportunity for human error in which the pages get mixed up and the wrong calibration(s) result. A method of reducing such error is through using patch codes, which is described, for example, in U.S. Pat. No. 6,972,867 to Venable, et al., entitled, PATCH CODES FOR COLOR CALIBRATION JOB IDENTIFICATION ENCODING. Typically one bit per CMY separation may be encoded per patch. Even with only one bit per separation, variations between printer models are sufficient to cause occasional read errors. One solution is to reduce the number of colors further (eliminating blue, which is too similar to black on some printers), or hand tune the colors to be more distinct.
US 2008/0204773, by Morgana and Klassen, describes an improved patch code method that uses a simple calibration step and RGB distance computations for patch code decoding. That method works well for well-behaved printers; however, sometimes fails when printers are in stress conditions. The exemplary embodiments presented herein are more robust to printer variation.
It would be desirable, therefore, to provide automation to the color printer calibration process, breaking the cycle where an operator must keep track of all details. It is further desirable that the job identification data be encoded according to a scheme wherein job identification data is printed according to a protocol and in a format approximately identical to a format of the target.